1. Field of the Invention
This invention relates broadly to semiconductor device structures. More particularly, this invention relates to semiconductor devices made with modulation doped quantum well interfaces that operate to detect high frequency electromagnetic radiation.
2. State of the Art
Satellite communication systems are now pushing into the sub-millimeter wave region in the quest for greater data throughput. With the upcoming wireless revolution and the anticipated demand for Internet services from almost anywhere, there will be significant demand for higher frequency carriers and modulation capability commensurate with these bandwidths. For future military terrestrial communication channels, the THz region (100 GHz-10,000 GHz) offers greatly enhanced capability. For satellite/ground and helicopter/ground links it avoids many of the problems of atmospheric scattering and absorption and adverse climatic conditions found in bandwidths up to 300 GHz. In addition, large toxic molecules of biological and chemical agents have resonances in the THz region and the detection of certain (chemical) weapons and explosives would be enabled. Furthermore, standoff detection (which involves passive and active methods for sensing of chemical and biological material when the sensor is physically separated from the site of interest) could be achieved by monolithic integration of a detector of electromagnetic radiation in the THz region (100 GHz-10,000 GHz), referred to herein as a THz detector, with data processing circuits providing a rugged, compact and portable sensor offering critical human protection.
State-of-the-art FET devices, such as High Electron Mobility Transistors (HEMTs), have conventional electronic cutoff frequencies in the 150-200 GHz range for channel lengths of 0.1 mm. Several reports discuss the possibility of using plasma resonance of the two-dimensional electron gas (referred to herein as “2D gas”) at the modulation doped interface in a HEMT to implement a THz detector. See i) W. Knap et al., “Resonant Detection of Subterahertz Radiation by Plasma Waves in a Submicron Field-Effect Transistor,” Applied Phys. Letts., Vol. 80, No. 18, May 6, 2002, pg. 3433-3436; ii) M. I. Dyakonov et al., “Plasma Wave Electronics: Novel Terahertz Devices using Two Dimensional Electron Fluid,” IEEE Trans. Elec. Dev., Vol. 43, No. 10, October 1996, pg. 1640-1645; and iii) M. Shur et al. “Terahertz Sources and Detectors Using 2D Electronic Fluid,” IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 4, April 2000, pg. 750. However, the mean free paths necessary for plasma resonances requires sub-0.1 μm transistor structures or cryogenic cooling to below 77 K. Such small transistor structures and cryogenic cooling substantially increases the manufacturing cost of the THz detector. Furthermore, there is no input/output isolation in the HEMT configuration since the channel must absorb the radiation as well as interface to another electronic device.
Other reports have suggested additional mechanisms by which the 2D gas at a modulation doped interface may function as a THz detector. In Barbieri et al., Hot-electron Multiquantum Well Microwave Detector Operating at Room Temperature,” Appl. Phys. Lett., Vol. 23, No. 2, 1995, pp 250-253 and Barbieri et al., “Broadband Microwave Detection with a Novel 2D Hot-Electron Device,” Superlattices and Microstructures, Vol. 23, No. 5, 1998, pp. 1079, a structure similar to a Quantum Well Infrared Photodetector (QWIP) device is used to generate a photocurrent due to incident electromagnetic radiation in the THz region. The mechanism of response is the thermionic emission of electrons from the confined state to the continuum due to electron heating of the 2D gas, i.e. the 2D gas was heated preferentially to the lattice. In K. S. Yngvesson, “Ultrafast two-dimensional electron gas detector and mixer for terahertz radiation,” Appl. Phys. Lett., Vol. 76, No. 6, February 2000, pp 777, thermionic emission of electrons from the confined state to the continuum due to electron heating of the 2D gas in a HEMT structure is used to produce a detector and mixer for THz radiation. The problem in utilizing such QWIP and HEMT device structures is that of extracting a useable signal from the device. More specifically, when the 2D gas is heated by an antenna connected thereto, there is no place for photocurrent to flow. In other words, there is no way to efficiently remove photocurrent from the device.